On the eve of the eight year anniversary of the 9/11 attacks, an FBI informant who infiltrated alleged terrorist cells in the U.S. tells ABC News the FBI missed a chance to stop the al Qaeda plot because they focused more on undercover stings than on the man who would later become known as 9/11 ringleader Mohammed Atta. Brian Ross reports on the undercover agent in al Qaeda. In an exclusive interview to be broadcast tonight on ABC World News with Charles Gibson and Nightline, former undercover operative Elie Assaad says he spotted and became suspicious of Atta in early...